The present invention relates generally to a fluid and method for fracturing oil well formations and, more specifically, to a cross-linking system for use with such a fracturing fluid to increase viscosity of the fluid.
In fracturing oil and gas wells, a special fluid referred to as a fracturing fluid is pumped down the well to contact the formation to be fractured. The fluid can contain proppant materials such as glass beads, nylon pellets, walnut shell fragments, or the like. The pressure of the fluid composition is increased by pumps located at the surface until the formation is fractured by the hydraulic pressure. The fracturing fluid is pumped at a rate sufficient to open a fracture in the exposed formation, and extend the fracture from the well bore into the formation. Continued pumping of the fracturing fluid containing a propping agent into the fracture results in proppant placement within the fractured zone. Following the treatment, the fracturing fluid is recovered from the well, leaving the proppant remaining in the fracture, thereby preventing the complete closure of the formation and forming a permeable channel extending from the well bore into the formation.
A fracturing fluid widely used in fracturing operations includes a gum such as a guar gum and water. When these two materials are mixed together in proper proportions, a viscous gel is formed. The guar gum is usually added to the water in the form of a dry powder and upon hydration forms gels of varying viscosities. The actual viscosity which is achieved in the resulting fracturing fluid is dependant upon the relative proportions of water and gum used. Other things being equal, the viscosity of the resultant fluid increases directly with the amount of additional gum used.
While adding increased amounts of gum to the base fluid is one technique for increasing the viscosity of the fluid, such a practice can become uneconomical because of the cost of the gum. A more efficient and economical practice involves the use of cross-linking agents. The cross-linking agents used in very small amounts, can give large increases in apparent viscosity of the base fluid. The cross-linkers work by chemically linking the linear polymers in the base fluid together, further restricting the ability of the water molecules to move. Some common cross-linking agents include borates, aluminum, antimony and titanium containing compounds including the so-called organotitanates. Each of the known cross-linking agents generally has certain desirable properties coupled with certain less than desirable properties.
The selection of the proper cross-linking agent is based upon the type of gelling agent being used, the pH of the gel system, the predicted fluid temperature, and other factors. Thus, in the case of borates, the pH of the base fluid needs to be in the range of about 8-10 for cross-linking, while with many of the other cross-linkers such as the metallic ions, the pH will need to be in the range of about 2-8, depending upon the base polymer used.
Another concern with the choice of cross-linking agent is the shear stability of the fluid while pumping down the tubular conduit and through the perforations into the formation. The shear stability can be evaluated in the laboratory the shearing the cross-linked gel with a shearing device and determining the time required for the cross-linked gel structure to reappear or reheal. Following shear, the cross-linked fluids should reheal rapidly under down hole conditions so propping agents will be transported and not dropped from the fluid inside the tubular conduit. The borate cross-linked system generally stands shear forces adequately for most fracturing applications while the organotitanate cross-linked fluids cannot take higher shear rates.
The temperature stability of the fracturing fluid is also of concern. Prolonged exposure of the fracturing fluid to the temperatures encountered in the well bore causes the fluid to "break" or lose viscous nature. Many fracturing fluids lose a large portion of their viscosity on heating to 200.degree. F. and a majority lose their viscosity upon heating to 300.degree. F. It is necessary for the fracturing fluid to remain viscous in the fracture long enough to permit build-up and maintenance of sufficient pressure to open a fracture. Additionally, the viscous fracturing fluid can support propping agents suspended therein. A desirable fracturing fluid should retain its gelled or viscous nature for up to 8 hours in the temperature range of 235.degree.-300.degree. F. The organotitanate cross-linked fluids are temperature stable in the above range but are shear sensitive. The borate cross-linked fluids while shear stable tend to be temperature sensitive.